The present invention relates to a plasma generating apparatus using a high-frequency electric discharge.
A plasma generating method using a high-frequency electric discharge is used in various fields such as dry etching for microprocessing, sputtering and plasma CVD for forming thin films, and an ion implantation apparatus. In such a method, it is required to generate a plasma under high, vacuum in order to provide extremely fine features and to control film quality with high precision.
Below, a dry-etching technique suitable for microprocessing will be described as an exemplary application of the plasma generating method.
The recent progress in the field of highly dense semiconductor integrated circuits is bringing about changes comparable to those brought about by the Industrial Revolution. Higher densities have been attained in semiconductor integrated circuits by reducing element dimensions, improving devices, providing chips with larger areas, and the like. Element dimensions have been reduced to the extent of the wavelength of light. In lithography, the use of excimer laser or x-rays is promising. In forming micropatterns, dry etching as well as lithography are playing important roles.
Dry etching is a processing technique for removing unnecessary portions of a thin film or substrate by utilizing a chemical or physical reaction at the interface between a solid phase and a gas phase of radicals, ions and the like, which are present in a plasma. Reactive ion etching (RIE) is most widely used among dry-etching techniques. According to RIE, a sample is exposed to a plasma of an appropriate gas, which has been generated by a high-frequency electric discharge, so that the resultant etching reaction removes unnecessary portions of the sample on its surface. The necessary portions that should not be removed are generally protected by a photoresist pattern serving as a mask.
In microprocessing, it is required for ions to move in the same direction, and for this purpose, it is indispensable to reduce the scattering of the ions in the plasma. To equalize the direction of ionic movements, it is effective to increase the mean free path of the ions by reducing the pressure in a plasma generating apparatus. However, this causes a problem that a high-frequency electric discharge hardly occurs under high vacuum in a plasma chamber.
In view of the foregoing, there have been developed magnetron RIE, ECR dry etching, and like techniques which facilitate the generation of an electric discharge by applying a magnetic field to a plasma chamber.
FIG. 13 is a view diagrammatically showing a conventional ion-etching apparatus using a magnetron discharge. In a metallic chamber 51 is introduced a reactive gas via a gas controller 52. The pressure in the chamber 51 is controlled at an appropriate value by an exhaust system 53. In the upper part of the chamber 51 is provided an anode 54. In the lower part of the chamber 51 is provided a sample stage 55 serving as a cathode. The sample stage 55 is connected to a RF power supply 57 via an impedance-matching circuit 56 so that a high-frequency electric discharge is generated between the sample stage 55 and the anode 54. On the side parts of the chamber 51 are provided two pairs of AC electromagnets 58, each of which differs in phase by 90.degree.. A rotating magnetic field is applied into the chamber 51 by the two pairs of AT electromagnets 58, thereby facilitating the generation of an electric discharge under high vacuum. This causes the cycloid movement of electrons, which lengthens the path traveled by an electron, resulting in higher ionization efficiency.
FIG. 14(a) shows an embodiment in which boron phosphorus glass is etched by a conventional magnetron RIE or ECR dry-etching apparatus. In the drawing, 60 designates a Si substrate, 61 designates the boron phosphorus glass, and 62 designates a photoresist pattern.
However, such a conventional apparatus as mentioned above is disadvantageous in that a device may be damaged as will be described below. With the conventional magnetron RIE apparatus, the rotating magnetic field dissolves the uneven distribution of a plasma to be averaged with the passage of time. However, since the momentary strength of the magnetic field differs radially in the chamber, as shown in FIG. 14(b), the plasma density differs accordingly from one place to another, so that non-uniform etching may be performed or potential difference may be caused locally. When the conventional magnetron RIE apparatus is applied to a MOSLSI process, therefore, the gate oxide film may be destroyed disadvantageously.
Similarly in the ECR apparatus, the momentary strength of the magnetic field generally differs in the radial direction of the chamber, so that non-uniform etching may be performed or potential difference may be caused locally due to the uneven density of the plasma, which is disadvantageous.
In view of the foregoing, Japanese Patent Application No. 2-402319 proposes a plasma generating apparatus comprising: a vacuum chamber; plasma generating means including N first electrodes (N is an integer not less than 2) which are disposed at roughly regular intervals in the vacuum chamber and a first high-frequency power supply for supplying said first electrodes in the order of their arrangement with high-frequency electric powers, each of which has a first frequency but differs in phase by (360/N) degrees, so as to generate, by means of a rotating electric field formed by said first electrodes, a highly dense plasma in a plasma generating part surrounded by said first electrodes; and ion extracting means including a second electrode, an earth electrode, both of which are disposed in said vacuum chamber, and a second high-frequency power supply for supplying said second electrode with high-frequency electric power having a second frequency so as to extract ions from the plasma which has been generated in said plasma generating part.
Though this plasma generating apparatus can generate a highly dense and uniform plasma under high vacuum, an object of the present invention is to provide a plasma generating apparatus which can generate a more uniform plasma, which is suitable for microprocessing, and which minimizes damage to a device.